In 2009, physicist Stephen Hawking held a party for time travelers. He announced it 3 years later to ensure that only people with the ability to travel back in time could attend. No one showed up.

Robert Nemiroff, a physicist at Michigan Technological University, and graduate student Teresa Wilson also looked for evidence of time travelers at work in the world. They recently published a paper on the results of their search. To find time travelers, they looked for evidence of online content that only people who could travel backward in time would be able to create:

They looked for signs of foreknowledge about specific events on the Internet, or in Twitter tweets. The two events they chose were the discovery of Comet ISON in September 2012, and the selection of Pope Francis in March 2013. Looking for the evidence was trickier than it might sound: For example, search engines such as Google and Bing can misinterpret time stamps for Web pages, and it's also possible to back-date blog entries or Facebook posts.

The researchers found Twitter searches to be the most reliable tools, because tweets can't be back-dated. "No clearly prescient content involving 'Comet ISON,' '#cometison,' 'Pope Francis' or "#popefrancis' was found from any Twitter tweet — ever," they reported.

They also issued a request for time travelers to send tweets using either the hashtag "#ICanChangeThePast2" or "#ICannotChangeThePast2" by the end of August 2013. The trick was that the request was issued in September. No such tweets were found that predated the deadline. (Some have been received since, however, including this one from Thursday: "Your future laws forbid me from giving you any proof of our technologies. But be sure that we're watching you. #ICanChangeThePast2")

Dr. Nemiroff and Ms. Wilson are trying to get their paper published in a scholarly journal. So far, they have not been successful:

Nemiroff told NBC News that the time-travel paper was submitted to three journals, but was rejected by all three. "At this point, I don't know if it will ever appear in a refereed journal," he said. If there are any time travelers out there who can let him know, he'd appreciate hearing about it. And while you're at it, he'd also appreciate receiving "a large sum of money in negotiable currency."